Table 1. Alloy Factors for the Calculation of Ideal Diameter

Question:
I am investigating sinter hardening and want to understand the role that Mn, Cr and Si play in hardenability since they are lower-priced alternatives to say Ni or Mo. What alloy elements are better in a PM grade? I seem to recall that there is an equation that relates the effect of different alloys additions. Can you help me find it?

Answer:
Steels can be assessed in terms of their carbon equivalent (CE), which scales the concentration of each element by its ability to retard the austenite/martensite transformation. It is also used to express the hardenability of alloy steels in terms of equivalent plain-carbon steel. It is calculated as follows:

(1) CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

In addition, ideal-diameter calculations can be used to compare the relative effect of alloying elements. Ideal diameter can be calculated fairly precisely having knowledge of the austenite grain size and alloying elements present in the steel of interest. ASTM Specification A255-02 (Standard Test Method for Determining Hardenability of Steel) assumes a grain size of 7 in its calculation since, statistically, most steels are close to this grain-size value.

Let’s calculate the DI value for a given steel with an ASTM No. 8 grain size and the following chemical composition: 0.30%C, 0.20%Si, 0.40%Mn, 0.15%Ni, 0.95%Cr, 0.15%Mo. The ideal diameter is calculated using Equation (2) where the individual values can be found in Table 1.

(2) DI = DI jominy x fMN x fSI x fNI x fCR x fMO

Our calculation then becomes:

(3) DI = DI jominy x fMN x fSI x fNI x fCR x fMO
= 0.17 x 1.14 x 2.33 x 1.055 x 3.052 x 1.45
= 2.10 inches (53.34 mm)

Recall that the hardenability of steel depends upon the actual rate at which its austenite transforms either to fine pearlite or martensite. The precise temperature at which this transformation rate is greatest depends upon the composition of the steel. In any case, it is this maximum rate at temperatures near 500ºC (950ºF), which determines the critical quenching speed that must be exceeded if the steel is to be hardened.

This maximum rate of austenite transformation to products other than martensite is, in turn, largely determined by the condition of the austenite at the moment of quenching with respect to two factors:

1. Composition – With respect to dissolved elements, most of which retard, though some may hasten, its transformation to pearlite. Manganese, chromium, nickel, silicon and aluminum definitely retard the reaction and so contribute directly to deep hardening – in the order named. On the other hand, tungsten, cobalt, molybdenum, vanadium and possibly oxygen appear to induce shallow hardening, though most probably indirectly by restricting grain growth.

2. Effective grain size – The finer the grain, the more rapid is the transformation to fine pearlite, and correspondingly, the lower is the hardenability. Effective grain size seems to be the most potent single factor influencing hardenability. It, in turn, is probably controlled largely (for any specified temperature) by the obstruction to grain growth offered by large numbers of very finely dispersed particles (of exceedingly small aggregate mass) comprised presumably of stable oxides such as alumina, vanadium and (probably) silica or silicates.